AlGaInP-BASED SEMICONDUCTOR LASER

ABSTRACT

An aluminium gallium indium phosphide (AlGaInP)-based semiconductor laser device is provided. On a main surface of a semiconductor substrate formed of n-type GaAs (gallium arsenide), from the bottom layer, an n-type buffer layer, an n-type cladding layer formed of an AlGaInP-based semiconductor containing silicon (Si) as a dopant, an active layer, a p-type cladding layer formed of an AlGaInP-based semiconductor containing magnesium (Mg) or zinc (Zn) as a dopant, an etching stopper layer, and a p-type contact layer are formed. Here, when an Al composition ratio x of the AlGaInP-based semiconductor is taken as a composition ratio of Al and Ga defined as (Al x Ga 1-x ) 0.5 In 0.5 P, a composition of the n-type cladding layer is expressed as (Al xn Ga 1-xn ) 0.5 In 0.5 P (0.9&lt;xn&lt;1) and a composition of the p-type cladding layer is expressed as (Al xp Ga 1-xp ) 0.5 In 0.5 P (0.9&lt;xp≦1), and xn and xp satisfy a relationship of xn&lt;xp.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and is a continuation of U.S.application Ser. No. 13/714,508 filed on Dec. 14, 2012, which claimspriority from Japanese Patent Application No. 2012-004283 filed on Jan.12, 2012. The content of each of the above documents are incorporated byreference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a semiconductor laser. Moreparticularly, the present invention relates to technique effectivelyapplied to a semiconductor laser using an aluminium gallium indiumphosphide (AlGaInP)-based semiconductor.

BACKGROUND OF THE INVENTION

Japanese Patent Application Laid-Open Publication No. 2002-217495(Patent Document 1) discloses technique of suppressing impuritydiffusion to an active layer and also increasing characteristictemperature and modulation frequency for a semiconductor laser in whichat least an n-type cladding layer, a bottom optical wave guiding layer,an active layer, a top optical guiding layer and a p-type cladding layerare stacked on a semiconductor substrate.

More specifically, by using Al_(0.5)In_(0.5)P which is lattice-matchedto a gallium arsenide (GaAs) substrate and having the largest bandgapamong AlGaInP-based semiconductors to the p-type cladding layer and then-type cladding layer to obtain a bandgap difference between the activelayer and the cladding layers, electron overflow from the active layerto the p-type cladding layer is suppressed. Also, diffusion of a dopant(zinc (Zn), selenium (Se)) doped to the p-type cladding layer and then-type cladding layer at a high concentration, i.e., 1×10¹⁸ cm⁻³ intothe active layer is suppressed by providing an undoped layer between theactive layer and the p-type cladding layer and between the active layerand the n-type cladding layer, respectively.

Japanese Patent Application Laid-Open Publication No. 2006-120968(Patent Document 2) discloses technique of improving efficiency andtemperature characteristics of a semiconductor laser having an activelayer between an n-type cladding layer and a p-type cladding layer. Morespecifically, according to Patent Document 2, generation of misfitdislocation is suppressed by forming the p-type cladding layer and then-type cladding layer with lattice-aligned Al_(0.5)In_(0.5)P andintroducing a strained layer for inhibiting overflow of electronsbetween the active layer and the p-type cladding layer as well as makingthe thickness of the strained layer smaller than or equal to a criticalthickness.

Japanese Patent Application Laid-Open Publication No. 2011-023493(Patent Document 3) discloses technique of reducing catastrophic opticaldamage (COD) at end facets and also improving stability of output beamfor an AlGaInP-based semiconductor laser of a horizontal cavity typehaving a lasing wavelength shorter than 650 nm. More specifically,according to Patent Document 3, dissipation of an optical waveguidestructure near end facets occurring when a window structure is formed bydiffusion of an impurity such as Zn near end facets is suppressed byforming the optical waveguide layer excluding a well layer with(Al_(x)Ga_(1-x))InP where x>0.66. In addition, in the semiconductorlaser, the n-type cladding layer and the p-type cladding layer areformed of Al_(0.51)In_(0.49)P or (Al_(0.9)Ga_(0.1))_(0.51)In_(0.49)P.

Note that, a band gap of an AlGaInP-based semiconductor is 2.3 eV whenx=0.7 in (Al_(x)Ga_(1-x))_(0.5)In_(0.5)P. However, it is reported thatthe bandgap is 2.35 eV when x=1 and it is the largest value amongAlGaInP-based semiconductors (see D. P. Bour, R. S. Geels, D. W. Treat,T. L. Paoli, F. Ponce, R. L. Thornton, B. S. Krusor, R. D. Bringans, D.F. Welch (1994). “Strained GaxIn1-xP/(AlGa) 0.5In0.5P heterostructuresand quantum-well laser diodes”, IEEE Journal of Quantum Electronics—IEEEJ QUANTUM ELECTRON, vol. 30, no. 2, pp. 593-607 (Non-Patent Document3)).

SUMMARY OF THE INVENTION

Application of a red semiconductor laser used as a light source for DVDsto small-size projectors laser displays such as a red light source hasbeen advanced.

When using a red semiconductor laser as a light source for projectors,high-temperature operation and high-optical output power operation forcorresponding to improvements in luminance of projectors or improvementsin luminosity factor by shortening the wave length are required. Inaddition, the red semiconductor laser has been required to correspond tohigh-temperature operation eyeing usages for mobile devices and in-carusages.

However, semiconductor lasers have problems of difficulties in achievinggood high-temperature and high-optical output power characteristics dueto significant influences of electron overflow into a p-type claddinglayer from an active layer caused by a temperature increase in avicinity of the active layer upon high-temperature and high-opticaloutput power operation.

Existing lasing wavelengths of red semiconductor lasers for DVDs arearound 660 nm. In comparison, lasing wavelengths around 640 nm arerequired of red semiconductor lasers for displays. However, the shorterthe lasing wavelengths are, the smaller the bandgap difference betweenthe active layer and the p-type cladding layer is and the moresignificant carrier overflow upon high-temperature operation is.Therefore, achieving improvements in high-temperature characteristics isa problem.

Generally, one of the characteristics of AlGaInP-based materials is thatthe larger the Al composition ratio is, the larger the bandgap is andthe lower the refractive index is. Therefore, in view ofhigh-temperature characteristics, it is preferable to make the Alcomposition ratio of the cladding layer as large as possible. However, aproblem in reliability occurs such that the larger the Al compositionratio is, the lower the crystallinity is and the more significantdiffusion of dopant is. Accordingly, the Al composition ratio of thecladding layer has been set at about 0.6 to 0.7 for the redsemiconductor lasers having lasing wavelengths around 660 nm, inconsideration of high-temperature characteristics and reliability.

However, as the carrier overflow at high temperature is more significantin the red semiconductor lasers having lasing wavelengths around 640 nm,it is required to further improve high-temperature characteristics andthus it is desirable to make the Al composition of the cladding layerratio as large as possible. Therefore, when AlInP having the largest Alcomposition ratio is used as material of the cladding layer, the bandgapdifference between the active layer and the cladding layer is maximumand also the refractive index of the cladding layer is minimum and thusthere is a merit that optical confinement in the active layer can belarge.

As the technique disclosed in Patent Document 1 described above,technique of improving temperature characteristics by using AlInP in thep-type cladding layer and the n-type cladding layer and of preventingdopant diffusion by providing undoped layers between the active layerand the p-type cladding layer and between the active layer and then-type cladding layer has been known.

However, as described below, the inventors of the present invention haveexamined reliability of a red semiconductor laser device using a lasingwavelength of 640 nm in which AlInP is used in a p-type cladding layerand an n-type cladding layer and the reliability was thousand-hour scaleas a result. Although the reliability is at a practical level forgeneral usages of semiconductor lasers, it has been found out that usingAlInP in the p-type cladding layer and the n-type cladding layer cannotsufficiently correspond to high requirements in reliability, i.e., tenthousand hours or longer that is required of light sources for laserdisplays.

A preferred aim of the present invention is to provide technique capableof achieving both improvements in high-temperature characteristics andimprovements in reliability of semiconductor laser devices usingAlGaInP-based semiconductors.

The above and other preferred aims and novel characteristics of thepresent invention will be apparent from the description of the presentspecification and the accompanying drawings.

The typical ones of the inventions disclosed in the present applicationwill be briefly described as follows.

A semiconductor laser of a preferred embodiment of the present inventionis provided with a semiconductor laser including: an n-type claddinglayer having a composition of (Al_(xn)Ga_(1-xn))_(0.5)In_(0.5)P where0.9<xn<1; a p-type cladding layer having a composition of(Al_(xp)Ga_(1-xp))_(0.5)In_(0.5)P where 0.9≦xp≦1; and an active layerprovided between the n-type cladding layer and the p-type claddinglayer, in which a relationship of an Al composition ratio xn of then-type cladding layer and an Al composition ratio xp of the p-typecladding layer satisfies xn<xp.

The effects obtained by typical aspects of the present invention will bebriefly described below.

Both improvements in high-temperature characteristics and improvementsin reliability of semiconductor laser devices using AlGaInP can beachieved.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a configuration of a mainpart of a semiconductor laser device according to a first embodiment ofthe present invention;

FIG. 2 is a graph illustrating a result of calculating opticalconfinement Γ into an active layer where a composition of a p-typecladding layer is fixed and an Al composition ratio of an n-typecladding layer is changed;

FIG. 3 is a graph illustrating a calculation result of an FFP horizontalhalf width;

FIG. 4 is a graph illustrating I-L shapes at high temperature of thefirst embodiment and a comparative example;

FIG. 5 is graph illustrating lifetime test results of the firstembodiment and the comparative example;

FIG. 6 is a graph illustrating measurement results of photoluminescencewavelengths from active layers of the first embodiment and thecomparative example;

FIG. 7 is a graph illustrating a relationship of a Al composition ratioand fluctuations of an FFP horizontal half width and a relationship ofthe Al composition ratio and a kink level of the n-type cladding layer;

FIG. 8 is a broken perspective view of main parts illustrating a wholeconfiguration of the semiconductor laser device according to the firstembodiment of the present invention;

FIG. 9 is a broken perspective view of main parts illustrating a wholeconfiguration of a semiconductor laser device according to a secondembodiment of the present invention; and

FIG. 10 is a cross-sectional view illustrating a configuration of a mainpart of a semiconductor laser device according to a third embodiment ofthe present invention.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

Note that components having the same function are denoted by the samereference symbols throughout the drawings for describing the embodiment,and the repetitive description thereof will be omitted.

First Embodiment

A first embodiment is applied to a red semiconductor laser having alasing wavelength of 640 nm. FIG. 1 is a cross-sectional viewillustrating a configuration of a main part (laser chip) in thesemiconductor laser according to the present embodiment.

As illustrated in FIG. 1, a laser chip 10A includes a semiconductorsubstrate 10 formed of n-type GaAs (gallium arsenide). On a main surfaceof the semiconductor substrate 10, an n-type buffer layer 11, an n-typecladding layer 12, an active layer 13, a p-type cladding layer 14, anetching stopper layer 15, and a p-type contact layer 17 are formed inthis order from the bottom.

The n-type buffer layer 11 is formed of GaAs containing Si (silicon) asa dopant. The n-type cladding layer 12 is formed of AlGaInP (aluminiumgallium indium phosphide) containing Si (silicon) as a dopant. Here,when taking an Al composition ratio “x” (Al composition ratio of Al andGa in a compound semiconductor defined by(Al_(xn)Ga_(1-xn))_(0.5)In_(0.5)P) of the n-type cladding layer as “xn”,xn satisfies 0.9<xn<1, where xn=0.95, i.e.,(Al_(0.95)Ga_(0.05))_(0.5)In_(0.5)P in the present embodiment.

In addition, a Si concentration of the n-type cladding layer 12 is3×10¹⁷ cm⁻³ and a thickness of the n-type cladding layer 12 is 2.5 μm.When the dopant concentration of the n-type cladding layer 12 is low, anincrease in series resistance is posed and when the dopant concentrationis high, a lowering of slope efficiency due to an increase in internalloss is posed. Therefore, the dopant concentration which makes itpossible to prevent both an increase in series resistance and a loweringof slope efficiency is preferable to be set within a range from 1×10¹⁷cm⁻³ to 6×10¹⁷ cm⁻³.

The active layer 13 is formed in a multi quantum well (MQW) structure inwhich an optical guiding layer 13 a formed of AlGaInP, a well layer 13 bformed of GaInP or AlGaInP, a barrier layer 13 c formed of AlGaInP, anda well layer 13 d formed of GaInP or AlGaInP, and an optical guidinglayer 13 e formed of AlGaInP are stacked. A lowering in reliability ofthe active layer 13 due to dopant diffusion into the well layers 13 band 13 d can be prevented by making the above-mentioned layers 13 a to13 e undoped, respectively.

For efficiently confining light in the well layers 13 b and 13 d,thicknesses of the optical guiding layers 13 a and 13 e are 20 to 150 nmand Al composition ratios x of the optical guiding layers 13 a and 13 eare 0.4 to 0.8. For example, to obtain a beam spread angle in adirection perpendicular to the active layer 13 at 18°. (full width athalf maximum), the thickness is set at 25 nm and the Al compositionratio x is set at 0.7.

The well layers 13 b and 13 d can be given various lasing wavelengthsand oscillation modes by changing their thicknesses within a rangesmaller than or equal to a critical thickness, compositions and strains.When the well layers are unstrained or compressive strain is given, thethickness is preferably 3 to 6 nm and the Al composition ratio x ispreferably 0 to 0.15. For example, to set the lasing wavelength at 640nm, the thickness is 5 nm and the Al composition ratio x is 0.1 and acompressive strain of +0.8% is given. In this case, oscillation isgenerated in a TE mode in which electric-field components of the laserbeam are vibrated in a direction parallel to the active layer 13.

A thickness of the barrier layer 13 c is 5 to 10 nm and a composition ofthe barrier layer 13 c is the same as the optical guiding layers 13 aand 13 e. Note that, the composition of the barrier layer 13 c may bedifferent from that of the optical guiding layers 13 a and 13 e and astructure in which a plurality of the barrier layers 13 c sandwich thewell layers 13 b and 13 d may be used.

Note that, while a MQW structure in which the number of the well layersis two has been used here, a single quantum well (SQW) structure inwhich the number of the well layers is one may be used. When using thesingle quantum well (SQW) structure, since the volume of the well layeris reduced, it is easier to increase the carrier density inside the welllayer and a threshold gain can be obtained even when the injectioncurrent is small; therefore, there is an advantage that a thresholdcurrent for laser oscillation can be largely reduced.

The p-type cladding layer 14 is formed of AlGaInP containing Mg(magnesium) or Zn (zinc) as a dopant. Here, when taking an Alcomposition ratio x of the p-type cladding layer 14 as xp, xp satisfies0.9<xp≦1. In the present embodiment, xp=1, that is, Al_(0.5)In_(0.5)P.

A preferable dopant concentration of the p-type cladding layer 14 is1×10¹⁸ cm⁻³. In addition, a preferable thickness is 1.25 μm to besmaller than a thickness of the n-type cladding layer 12. To improvehigh-temperature characteristics and to reduce serial resistance, it ispreferable to make the dopant concentration of the p-type cladding layer14 as high as possible. However, when the Al composition ratio x ishigh, the dopant is prone to diffuse. Therefore, to prevent degradationof characteristics and lowering of reliability, it is preferable to setthe dopant concentration within a range of 6×10¹⁷ to 1.3×10¹⁸ cm⁻³.

When using an asymmetric structure in which the Al composition ratio xpof the p-type cladding layer 14 is higher than the Al composition ratioxn of the n-type cladding layer 12 like the present embodiment, theoptical distribution is biased toward the n-type cladding layer 12 side.Therefore, there is an effect of improving the slope efficiency whensetting the dopant concentration of the n-type cladding layer 12 lowerthan that of the p-type cladding layer 14. In addition, as to the typeof the dopant, it is preferable to use Mg which is less prone to diffusethan Zn. Moreover, although the thickness of the p-type cladding layer14 is preferable to be as small as possible for reducing seriesresistance, lowering of the slope efficiency is significant when thethickness is too small.

A ridge portion (ridge waveguide) 16 extending in a stripe pattern alonga direction perpendicular to the paper sheet is formed to the p-typecladding layer 14. A height of the ridge portion 16 is 1 μm and a widthof the ridge portion 16 is 2 μm. The ridge portion 16 is oscillated in asingle mode (single transverse mode). The p-type contact layer 17 formedof GaAs containing Zn as a dopant is formed to an upper portion of theridge portion 16.

In addition, the etching stopper layer 15 is provided to the p-typecladding layer 14 at a height that is about 0.25 μm from the activelayer 13. The etching stopper layer 15 is a layer for stopping etchingin the middle of forming the ridge portion 16 by etching the p-typecladding layer 14. The etching stopper layer 15 is formed of an AlGaInPlayer having a different Al composition ratio x than the p-type claddinglayer 14 for giving an etching selectivity with respect to the p-typecladding layer 14. The etching stopper layer 15 of the presentembodiment is formed of, for example, GaInP in which the Al compositionratio x is zero, and has a thickness of 3 nm. Note that, when etchingconditions of the p-type cladding layer 14 can be controlled well, theetching stopper layer 15 may be omitted.

A passivation film 18 formed of an insulating film such as silicon oxidefilm, silicon nitride film, or aluminum oxide film is formed to each ofsidewalls of the ridge portion 16 and the p-type contact layer 17 andplanar portions (upper surfaces of the etching stopper layer 15) in avicinity of both sides of the ridge portion 16.

Each of the semiconductor layers formed on the main surface of thesemiconductor substrate 10 is deposited by metal organic chemical vapordeposition (MOCVD). In addition, the passivation film 18 is formed bydepositing an insulating film on the semiconductor substrate 10 to whichthe p-type contact layer 17 is formed by CVD and then selectivelyetching only the insulating film at an upper portion of the p-typecontact layer 17 so that an upper surface of the p-type contact layer 17is exposed.

A p-side electrode 20 to be electrically connected to the p-type contactlayer 17 is formed to an upper portion of the passivation film 18. Onthe other side of the p-side electrode 20, an n-side electrode 21 isformed to a rear surface of the semiconductor substrate 10. Each of thep-side electrode 20 and the n-side electrode 21 includes an Au film andis formed of a metal film capable of making an ohmic contact.

The Al composition ratio xp of AlInP forming the p-type cladding layer14 is preferable to be as high as possible in view of high-temperaturecharacteristics and thus it is set at 1 at the maximum in the presentembodiment. Then, on that basis, the Al composition ratio xn of AlGaInPforming the n-type cladding layer 12 is set at 0.95.

In this manner, the semiconductor laser device of the present embodimenthas an asymmetric structure in which the Al composition ratio x differsin the n-type cladding layer 12 and the p-type cladding layer 14 and theAl composition ratio x of the p-type cladding layer 14 is higher thanthat of the n-type cladding layer 12. Therefore, penetration of light isbiased toward the n-type cladding layer 12 side where the refractiveindex is larger. There are advantages as follows.

More specifically, there is an effect of improving the slope efficiencybecause, when the penetration of light into the p-type cladding layer 14is reduced, it is possible to reduce light absorption in the p-typecontact layer 17 which is formed of GaAs that is an absorber of lighthaving a wavelength of 640 nm. In addition, as the present embodiment,when the dopant concentration of the n-type cladding layer 12 is madelower than that of the p-type cladding layer 14, more light isdistributed in the n-type cladding layer 12 having a lower dopantconcentration and thus it is possible to reduce internal loss occurringdue to free-carrier overflow in the cladding layers.

Reduction of the light penetration into the p-type cladding layer 14means enabling thickness reduction of the p-type cladding layer 14. Whenthe p-type cladding layer 14 is thinner, an effect of reducing heatgeneration amount by a reduction in series resistance and an effect ofimproving exhaust heat are achieved and thus high-temperaturecharacteristics are improved. Further, when the light distribution isbiased toward the n-type cladding layer 12 side, it is necessary toensure a refractive index difference by making the thickness of thep-type cladding layer 14 under the etching stopper 15 thinner forensuring a refractive index difference in a direction horizontal to theactive layer 13 (ensuring a same level of FFP horizontal half width).However, by reducing the thickness of the p-type cladding layer 14 underthe ridge portion 16, reactive current which does not contribute tolaser oscillation is reduced and thus threshold current can be reduced.

On the other hand, in the asymmetric structure in which the Alcomposition ratio x differs in the n-type cladding layer 12 and thep-type cladding layer 14, when the Al composition ratio differencebetween the n-type cladding layer 12 and the p-type cladding layer 14 isincreased, due to variations in compositions, the waveguide mode may beunstable and/or variations in characteristics may be increased. Further,when the Al composition ratio difference is extremely large so that apoint of the maximum light intensity is positioned at a boundary of theactive layer 13 and the cladding layers, even a fundamental mode is cutoff and laser oscillation is not permitted. Therefore, there is an upperlimit in the Al composition ratio difference between the n-type claddinglayer 12 and the p-type cladding layer 14.

Accordingly, the upper limit in the Al composition ratio difference ofthe n-type cladding layer 12 and the p-type cladding layer 14 wasexamined by a calculation. FIG. 2 illustrates a result of a calculationof optical confinement coefficient Γ. of optical confinement into theactive layer 13 where the composition of the p-type cladding layer 14 isfixed to Al_(0.5)In_(0.5)P and the Al composition ratio xn of the n-typecladding layer 12 is varied based on the structure of the presentembodiment described above. For example as shown in FIG. 2 xn has thevalues of 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99 and 1.

The smaller the Al composition ratio xn of the n-type cladding layer 12,the smaller the optical confinement coefficient Γ. When the Alcomposition ratio xn is 0.9, light does not enter the active layer 13 ina non-ridge portion and thus laser oscillation is not permitted.Therefore, it is understood that the Al composition difference ofthen-type cladding layer 12 and the p-type cladding layer 14 is requiredto be smaller than 0.1. Here, when a lowering rate of opticalconfinement having the Al composition ratio x=0.92 is converted intothreshold current, the lowering rate corresponds to a lowering by 20% ormore. An increase in threshold current poses an increase in reactivecurrent and it lowers high-temperature characteristics and reliability.Therefore, it is preferable to set the increased amount to 20% orsmaller, that is, it is preferable to set the Al composition ratiodifference to 0.08 or smaller. It is especially preferable to set the Alcomposition ratio difference to 0.05 or smaller for increasing opticalconfinement when the Al composition ratio xp is 1, as shown in FIG. 2.

The calculation result explained above means that light is not permittedto enter the ridge portion 16 abruptly as the Al composition ratio xn ofthe n-type cladding layer 12 is decreased and it largely affects the FFPin the horizontal direction among other characteristics values.Accordingly, influence on the beam spread angle in the FFP horizontaldirection given by the Al composition ratio difference was furtherexamined. FIG. 3 illustrates a calculation result of FFP horizontal halfwidth.

When the inventors of the present invention confirmed variations in Alcomposition ratio x among wafers, it was found that there was avariation of about .+−.0.01 from a target value. Accordingly, assumingthat a variation is .+−.0.01 in mass production, the influence on theFFP horizontal half width is illustrated with the case of shifting theAl composition ratio x by +0.01 and the case of shifting the Alcomposition ratio x by −0.01 together in FIG. 3. Accordingly anapproximate value of the composition ratio xn or xp is accurate towithin +/−0.01. In other words an approximate xn or xp value of 0.95would be between 0.94 and 0.96.

As illustrated in FIG. 3, the smaller the Al composition ratio xn of then-type cladding layer 12, that is, the larger the Al composition ratiodifference from the p-type cladding layer 14, the larger the influenceon the FFP horizontal half width. In addition, in accordance withcustomer's demands, that is, restrictions in optical design, it isnecessary to suppress the FFP horizontal half width from 6 to 12degrees, that is, a range within 6 degrees. To achieve that, inconsideration of fluctuations in the Al composition ratio x in massproduction, the Al composition ratio difference of the n-type claddinglayer 12 and the p-type cladding layer 14 is preferable to be smallerthan or equal to 0.08.

To confirm the effects of the present embodiment, the inventors of thepresent invention made a semiconductor laser and carried out anevaluation of characteristics and reliability on the semiconductorlaser. A wafer was cleaved with a cavity length of 2500 μm and apassivation film is used at end facets. To evaluate characteristics andreliability, a chip (laser chip) to which the semiconductor laser isformed was mounted on a sub-mount in the junction-down manner and thenthe sub-mount was loaded on a stem and the chip was hermetic sealed witha cap. Also, as a comparative example of the present embodiment, asemiconductor laser in which an n-type cladding layer and a p-typecladding layer are formed with Al_(0.5)In_(0.5)P was made.

FIG. 4 illustrates high-temperature I-L shapes of the present embodimentand the comparative example. The measurement conditions are 50° C. andCW. In the present embodiment, the threshold current was smaller thanthe comparative example by about 10% and the optical output power waslarger than the comparative example by 10% or more. In addition,although a kink was generated in the comparative example, a kink was notfound in the present embodiment and thus the I-L characteristic is goodin the present embodiment. The improvements in the high-temperaturecharacteristics in the present embodiment is considered to be achievedby an effect of an improvement in crystallinity of the active layer,which will be described in detail below, in addition to the effects of areduction in heat generation amount and an improvement in slopeefficiency by a reduction in threshold current.

FIG. 5 illustrates results of lifetime test of the present embodimentand the comparative example. Test conditions are 50° C. and 150 mWCW-APC. In the comparative example, an increased ratio of operationcurrent after 1500 hours was 2.5 to 4%. On the contrary, an increasedratio of operation current after 1500 hours was 0 to 1% in the presentembodiment. A reason of this result is considered as follows.

Generally, as Al is prone to be oxidized in property, when AlInP havinga high Al composition ratio x is made by crystal growth in MOCVD, oxygenand/or moisture in the air is absorbed on AlInP and it makescrystallinity to degrade easily. In addition, degradation incrystallinity in the n-type cladding layer influences quality of theactive layer which will be grown on the n-type cladding layer.Therefore, when AlInP is used in the n-type cladding layer like thecomparative example, a reason of the lowering in reliability occurred isconsidered such that dislocations threading the active layer wereincreased due to degradation in crystallinity in the n-type claddinglayer and multiplication of dislocations became more prone to occur whenthe laser element was operated for a long time. On the other hand, inthe present embodiment setting the Al composition ratio xn of the n-typecladding layer at 0.95, it is considered such that multiplication ofdislocations in the active layer was suppressed by an improvement incrystallinity in the n-type cladding layer.

FIG. 6 illustrates measurement results of photoluminescence wavelengthsfrom the active layers. While a full width at half maximum of thecomparative example described above was 18.3 nm, a full width at halfmaximum of the present embodiment was 11.7 nm and this is smaller thanthat of the comparative example. The full width at half maximumindicates quality of the active layer and the result means that thecomposition ratio or thickness has less fluctuations in thephotoluminescence-observed region having a diameter of about 2.5 mm. Asto a thickness in the wafer plane upon finishing crystal growth,fluctuations were suppressed to 1% or less in both the presentembodiment and the comparative example. Therefore, the change in thefull width at half maximum is supposed to be caused by a fluctuation inelemental composition ratio.

More specifically, in the comparative example, it is considered that Aland In which are group III elements locally formed Al-rich regionsand/or In-rich regions so that fluctuations in composition ratio occur.Meanwhile, in the present embodiment, it is considered that introductionof Ga brought an effect of averaging fluctuations in composition ratio.More specifically, it is supposed that Ga is easy to be mixed in thelocal Al-rich regions and/or In-rich regions as Ga has an atomic radiusbetween those of Al and In and thus local Al-rich regions and/or In-richregions are easier to dissipate as a result. When the local Al-richregions are dissipated, regions in which local absorption of oxygenand/or moisture often occurs are reduced. Therefore, it is supposed thatdislocations by concentration of dislocations and further multiplicationof dislocations could be suppressed and it resulted in the highreliability. That is, by using AlGaInP in which a minute amount of Gawas added in the n-type cladding layer, the crystallinity of the n-typecladding layer was improved and it was lead to the improvement incrystallinity in the active layer formed on the n-type cladding layerand thus the effects of improvements in characteristics and reliabilitywere exhibited.

As a modification example of the present embodiment, the n-type claddinglayer may be formed of (Al_(0.92)Ga_(0.08))_(0.5)In_(0.5)P (Alcomposition ratio xn=0.92) and the p-type cladding layer may be formedof Al_(0.5)In_(0.5)P (Al composition ratio xp=1). In this case, the Alcomposition ratio difference between the cladding layers are larger andthus variations in FFP horizontal half width among wafers will be largein mass production. However, as described in the calculation example inFIG. 3 above, the present modification example is the lower limit thatdoes not pose a problem in view of the standard of the FFP horizontalhalf width.

When the Al composition ratio difference between the n-type claddinglayer and the p-type cladding layer is large, as described above, thereis a merit of high-temperature characteristics because the p-typecladding layer can be made thinner etc. However, on the other hand,there is such a tradeoff relationship that there is a demerit of anincrease in characteristics variations when the Al composition ratiodifference is too large.

FIG. 7 illustrates a relation of an Al composition ratio xn of then-type cladding layer and a variation of FFP horizontal half width and arelation of an Al composition ratio xn of the n-type cladding layer anda kink level (optical output power value in which a kink occurs). Here,the variation of FFP horizontal half width is a value calculated fromthe calculation result in FIG. 3 and the value is obtained by dividing adifference between the half widths of the variations .+−.0.01 of the Alcomposition ratio (xn) by a half width at the center. In addition, thekink level is obtained from actual measured values of the presentembodiment and the comparative examples.

A lowering of kink level and an increase in variation of FFP horizontalhalf width pose an increase in a cost due to a lowering of yield.Therefore, to consider mass production, a difference (xp-xn) between theAl composition ratio (xp) of the p-type cladding layer and the Alcomposition ratio (xn) of the n-type cladding layer is preferable to be0.02 or larger.

Also, as a modification example of the present embodiment, aconfiguration in which As is contained in the active layer can be used.For example, an optical guiding layer is formed of AlGaAs and a welllayer is formed with GaAs and a thickness of the well layer is 6 nm. Inthis case, a near infrared light around 0.83 μm lasing wavelength can beobtained. Generally, near infrared and infrared semiconductor lasers of0.7 nm to 1 μm lasing wavelength have a cladding layer formed of AlGaAs.On the contrary, when the cladding layer is formed of an AlGaInP-basedmaterial, a bandgap difference between the cladding layer and the activelayer can be large and thus an improvement in high-temperaturecharacteristics is possible.

FIG. 8 is a broken perspective view of main parts illustrating a wholeconfiguration of the semiconductor laser of the present embodiment. Thesemiconductor laser includes: a stem 30 in a disk-like shape formed of aFe (iron) alloy and having a diameter of about 5.6 mm and a thickness ofabout 1.2 mm; and a package (sealing container) having a cap 31 forcovering an upper surface of the stem 30. An outer perimeter of a bottomportion of the cap 31 is fixed to the upper surface of the stem 30. Inaddition, a round hole 33 to which a glass plate 32 permeable to laserbeam is joined is provided to a center portion of the upper surface ofthe cap 31.

A heat sink 34 formed of a metal having a good heat conductivity ismounted in a vicinity of a center of the upper surface of the stem 30covered by the cap 31. The heat sink 34 is joined to the upper surfaceof the stem via a brazing filler metal (not illustrated) and a submount35 is fixed to the whole surface of the upper surface of the stem 30 viasolder (not illustrated). On the other side, to the lower surface of thestem 30, three leads 40, 41 and 42 are attached.

A laser chip 10A illustrated in FIG. 1 is mounted to a chip-mountingsurface of the submount 35 in the junction-down manner. The submount 35both works as a heat dissipation plate for dissipating heat generatedupon light emission of a laser beam to the outside of the laser chip 10Aand a supporting substrate for supporting the laser chip 10A.

To the chip-mounting surface of the submount 35, a submount electrode(not illustrated) electrically connected to the p-side electrode 20 (seeFIG. 1) of the laser chip 10A is formed and an end of an Au wire 36 isbonded to a surface of the submount electrode. On the other side, an endof an Au wire 37 is bonded to a surface of the n-side electrode 21 ofthe laser chip 10A.

A laser beam exits from both end facets (upper end facet and lower endfacet in FIG. 8) of the laser chip 10A mounted on the submount 35. Thus,the submount 35 supporting the laser chip 10A is fixed to the heat sink34, so that the chip-mounting surface faces a direction perpendicular tothe upper surface of the stem 30. The laser beam (forward light) exitedfrom the upper end facet of the laser chip 10A exits to the outsidethrough the round hole 33 of the cap 31.

Second Embodiment

In a second embodiment, in addition to an improvement inhigh-temperature characteristics and an improvement in crystallinitydescribed in the first embodiment, an effect of an improvement inhumidity resistance according to a high-temperature and high-humiditytest will be described.

In a semiconductor laser of the second embodiment, an n-type claddinglayer is formed of (Al_(0.91)Ga_(0.09))_(0.5)In_(0.5)P (Al compositionratio xn=0.91) and a p-type cladding layer is formed of(Al_(0.96)Ga_(0.04))_(0.5)In_(0.5)P (Al composition ratio xp=0.96) andthe other configuration of the semiconductor laser is the same as thatof the first embodiment.

Normally, a semiconductor laser is housed in a package hermetic sealedwith a cap and the inside of the cap is maintained in atmosphere likedry air in which moisture is eliminated as much as possible. However, ifimplementing open-packaging of the semiconductor laser is possible,there are great merits in downsizing and price reduction.

Accordingly, as illustrated in FIG. 9, the laser chip 10A to which thesemiconductor laser having the configuration described above was mountedon the submount 35 in the junction-down manner and subjected to ahigh-temperature and high-humidity test (85° C., 85% RH) without ahermetic sealing with a cap. In addition, the chip of the comparativeexample used in the first embodiment was mounted on the submount in thejunction-down manner and subjected to a high-temperature andhigh-humidity test (85° C., 85% RH) without a hermetic sealing with acap.

As a result, in the comparative example, there was no problem occurreduntil 1000 hours but there was a lowering in characteristics among thelaser elements (two in 22 laser elements tested) at 1500 hours. On theother hand, in the present embodiment, no laser element had a loweringin characteristics even after 1500 hours (zero in 22 laser elementstested).

As a result of a visual inspection of the laser element of thecomparative example which had a lowering in characteristics, there was adiscoloration in the p-type cladding layer at the end facet portions.That is, a reason of the lowering in characteristics in thehigh-temperature and high-humidity test is considered as corrosion by areaction of Al in the semiconductor crystal and moisture. On the otherhand, in the present embodiment, it is considered that introduction of aminute amount of Ga in semiconductor crystal and increased theresistance to corrosion reaction.

As described above, the upper surface of the p-type cladding layer iscovered with the passivation film and the p-side electrode and thus itis considered to be difficult for moisture to intrude from the uppersurface of the semiconductor substrate. However, since the end facetsare covered only by the passivation film, it is easy for moisture tointrude as compared with the upper surface. A conceivable countermeasureis that the passivation film is formed of a film having a high humidityresistance such as a silicon nitride film. However, there arerestrictions in the selection of the passivation film at the end facetsin view of reliability and thus desirable characteristics andreliability may not be achieved when the specie of the passivation filmis limited. Therefore, increasing the resistance of the semiconductorcrystal itself is effective in achieving both good characteristics andgood reliability.

Third Embodiment

In a third embodiment, the present invention is applied to a broad-areared semiconductor laser. FIG. 10 is a cross-sectional view illustratinga configuration of a main part (laser chip) of a semiconductor laserdevice of the present embodiment.

Generally, a broad-area semiconductor laser achieves higher opticaloutput power and higher heat generation than a ridge semiconductorlaser. Therefore, it is more effective to use an asymmetric structure inwhich an n-type cladding layer and a p-type cladding layer havedifferent Al composition ratios.

In the present embodiment, the n-type cladding layer 12 is formed of(Al_(0.95)Ga_(0.05))_(0.5)In_(0.5)P (Al composition ratio xn=0.95) andthe p-type cladding layer 14 is formed of Al_(0.5)In_(0.5)P (Alcomposition ratio xp=1). The active layer 13 is formed of a singlequantum well (SQW) structure including an optical guiding layer 13 aformed of AlGaInP, a well layer 13 b formed of GaInP or AlGaInP and anoptical guiding layer 13 e formed of AlGaInP.

As to the well layer 13 b, various lasing wavelengths and oscillationmodes can be given by changing its thickness within a range smaller thanor equal to a critical thickness, composition and strain. When a tensilestrain is given, the thickness is preferably 6 to 18 nm and the Alcomposition ratio x is preferably 0 to 0.15. For example, to obtain alasing wavelength of 640 nm, the thickness of the well layer 13 b is setto 15 nm, the Al composition ratio x of the well layer 13 b is x=0,i.e., GaInP is used, and the tensile strain is given at −0.8%. In thiscase, the laser is oscillated in a TM mode in which components of theelectric field of the laser beam are vibrated in a directionperpendicular to the active layer 13.

In addition, a width of a light emitting portion E can be 3 to 200 μm.When the width of the light emitting portion E is set like this, thelaser is oscillated in a multimode. The larger the width of the lightemitting portion R is, the larger the optical output power is obtained;however, as a threshold current is increased, a heat generation amountis increased and high-temperature characteristics are degraded. Forexample, to obtain an optical output power of 500 mW, the width of thelight emitting portion E is set at 50 μm. Formation of the lightemitting portion E may be done in a method of forming a ridge or amethod of opening a contact window for current injection.

Fourth Embodiment

In a fourth embodiment, in addition to the effect of improving humidityresistance described in the second embodiment, an effect of improvingability of mass production will be described.

In a semiconductor laser of the present embodiment, an n-type claddinglayer is formed of (Al_(0.95)Ga_(0.05))_(0.5)In_(0.5)P (Al compositionratio xn=0.95) and a p-type cladding layer is formed of(Al_(0.95)Ga_(0.05))_(0.5)In_(0.5)P (Al composition ratio xp=0.95).Except for this point, the semiconductor laser of the present embodimenthas the same configuration as the first embodiment. That is, in thesemiconductor laser of the present embodiment, the Al composition ratioxp of the p-type cladding layer and the Al composition ratio xn of then-type cladding layer are the same (xp=xn).

When the cladding layers are formed by MOCVD, a composition ratio of Aland Ga is adjusted by a flow rate of an organic metal gas. However, whenthe Al composition ratio is high like the present embodiment, since aflow rate of Ga is little and it is close to a limitation in accuracy ofa flow rate meter, variations in composition is large among wafers(wafers with different growth batches). According to a confirmation ofvariations in the composition ratio of Al and Ga (Al composition ratiox) among wafers, there were variations of about .+−.0.01 found withrespect to a target value. The result means that, in the case of thesecond embodiment, an actually formed n-type cladding layer may have anAl composition ratio xn=0.9 to 0.92 while a target value is xn=0.91, andan actually formed p-type cladding layer may have an Al compositionratio xp=0.95 to 0.97 while a target value is xp=0.96. Therefore, whenthe Al composition ratio x differs in the p-type cladding layer andn-type cladding layer like the second embodiment, not only variationsamong wafers but also variations in composition among the claddinglayers will occur, increasing variations in characteristics amongsemiconductor lasers and posing a yield lowering.

On the other hand, in the case of the fourth embodiment, since then-type cladding layer and p-type cladding layer can be successivelyformed with setting flow rate ratios of Al and Ga identical, there islittle difference in the Al composition ratio between the n-typecladding layer and the p-type cladding layer and thus what should beconsidered is only variations among wafers. Therefore, as compared withthe semiconductor laser in which the Al composition ratio x differs inthe p-type cladding layer and n-type cladding layer, variations incharacteristics can be suppressed and thus an improvement in yield andan improvement in ability of mass production can be achieved.

Also, as a modification example of the present embodiment, the n-typecladding layer can be formed of (Al_(0.91)Ga_(0.09))_(0.5)In_(0.5)P (Alcomposition ratio xn=0.91) and a p-type cladding layer is formed of(Al_(0.91)Ga_(0.09))_(0.5)In_(0.5)P (Al composition ratio xp=0.91). Whenthe Al composition ratio xp of the p-type cladding layer is reduced,high-temperature characteristics are lowered but humidity resistance isimproved. This case is on the assumption that the usage environment issevere and thus the Al composition ratio x can be set within the rangeof the present invention in accordance with a required usage.

While the first to fourth embodiments of the present invention made bythe inventors have been concretely described in the foregoing, to obtainthe effects of the present invention in view of high-temperaturecharacteristics, reliability and ability of mass production, the Alcomposition ratio (xp) of the p-type cladding layer is preferable to bein the range of 0.9<xp≦1 and the Al composition ratio (xn) of the n-typecladding layer is preferable to be in the range of 0.9<xn<1 and also adifference between xp and xn is preferable to be in the range of0≦xp−xn≦0.08.

Further, it is needless to say that the present invention is not limitedto the foregoing embodiments and various modifications and alterationscan be made within the scope of the present invention.

While the present invention has been applied to an AlGaInP-based redsemiconductor laser having a lasing wavelength of 640 nm for example inthe embodiments described above, since an AlGaInP-based semiconductorlaser can achieve a lasing wavelength of 0.6 μm to 0.7 μm by changingonly the design of the active layer, the present invention can beapplied to an AlGaInP-based semiconductor laser which is oscillated by alasing wavelength of 0.6 μm to 0.7 μm.

In addition, since a lasing wavelength of 0.7 μm to 1 μm can be achievedwhen As is contained in the active layer, the present invention can beapplied to near-infrared and infrared semiconductor lasers which areoscillated by a lasing wavelength of 0.7 nm to 1 μm in which anAlGaInP-based material is used in a cladding layer and also As iscontained in an active layer.

Moreover, the present invention can be applied to an array laser inwhich a plurality of semiconductor lasers are arrayed or a multi-beamlaser.

The present invention is applicable to AlGaInP-based semiconductorlasers.

What is claimed is: 1: A semiconductor laser chip comprising: an n-typecladding layer having a composition of (Al_(xn)Ga_(1-xn))_(0.5)In_(0.5)Pwhere 0.9<xn<0.98; a p-type cladding layer having a composition of(Al_(xp)Ga_(1-xp))_(0.5)In_(0.5)P where 0.92<xp<=1; an active layerprovided between the n-type cladding layer and the p-type claddinglayer, wherein the Al composition ratio xp of the p-type cladding layerand the Al composition ratio xn of the n-type cladding layer satisfies arelationship of xn<xp, wherein a difference between the Al compositionratio xp of the p-type cladding layer and the Al composition ratio xn ofthe n-type cladding layer satisfies a relationship of 0.02<=xp−xn<=0.08.2: The semiconductor laser chip of claim 1, where xp is approximately 1.3: The semiconductor laser chip according to claim 1, wherein a dopantconcentration of the p-type cladding layer is within a range from 6×10¹⁷cm⁻³ to 1.3×10¹⁸ cm⁻³. 4: The semiconductor laser chip according toclaim 1, wherein a dopant concentration of the n-type cladding layer iswithin a range from 1×10¹⁷ cm⁻³ to 6×10¹⁷ cm⁻³. 5: The semiconductorlaser chip according to claim 1, wherein a dopant concentration of thep-type cladding layer is higher than a dopant concentration of then-type cladding layer. 6: The semiconductor laser chip according toclaim 1, wherein a thickness of the p-type cladding layer is smallerthan a thickness of the n-type cladding layer. 7: The semiconductorlaser chip according to claim 1, wherein the active layer is formed ofGaInP or AlGaInP, includes one or two well layers having a thicknessfrom 3 nm to 6 nm, and is oscillated in a TE mode. 8: The semiconductorlaser chip according to claim 1, wherein the active layer is formed ofGaInP or AlGaInP, includes one well layer having a thickness from 6 nmto 18 nm, and is oscillated in a TM mode. 9: The semiconductor laserchip according to claim 1, further comprising: a submount mounting asemiconductor chip to which the semiconductor laser is formed in ajunction-down manner; a stem loading the sub mount; and a package incontact with the stem, in which the semiconductor chip is hermeticallysealed with a cap. 10: The semiconductor laser chip according to claim1, further comprising: a submount mounting a semiconductor chip to whichthe semiconductor chip is formed in a junction-down manner; and apackage in which the semiconductor chip is atmospherically exposed. 11:The semiconductor laser chip according to claim 1, wherein xn is 0.95.12: The semiconductor laser chip according to claim 1, wherein xn is0.96. 13: The semiconductor laser chip according to claim 1, wherein xnis 0.97. 14: The semiconductor laser chip according to claim 1, whereinxp is 0.95. 15: The semiconductor laser chip according to claim 1,wherein xp is 0.96. 16: The semiconductor laser chip according to claim1, wherein xp is 0.97. 17: The semiconductor laser chip according toclaim 1, wherein xp is 0.98. 18: The semiconductor laser chip accordingto claim 1, wherein xp is 0.99. 19: A semiconductor laser chipcomprising: an n-type cladding layer having a composition of(Al_(xn)Ga_(1-xn))_(0.5)In_(0.5)P where 0.92<=xn<1; a p-type claddinglayer having a composition of (Al_(xp)Ga_(1-xp))_(0.5)In_(0.5)P where0.95<=xp<=1; a ridge portion configured for optical confinement, locatedon the p-type cladding layer; an active layer provided between then-type cladding layer and the p-type cladding layer, an n-side electrodelocated on the n-type cladding layer; a p-side electrode located on theridge portion; and wherein the Al composition ratio xp of the p-typecladding layer and the Al composition ratio xn of the n-type claddinglayer satisfies a relationship of xn<xp. 20: A semiconductor laser chipcomprising: an n-type cladding layer having a composition of (Al_(xn)Ga_(1-xn))_(0.5) In_(0.5)P where 0.9<=xn<=0.92; a p-type cladding layerhaving a composition of (Al_(xp)Ga_(1-xp))_(0.5)In_(0.5)P where0.95<=xp<=0.97; a ridge portion configured for optical confinement,located on the p-type cladding layer; an active layer provided betweenthe n-type cladding layer and the p-type cladding layer, an n-sideelectrode located on the n-type cladding layer; a p-side electrodelocated on the ridge portion; and wherein a difference between the Alcomposition ratio xp of the p-type cladding layer and the Al compositionratio xn of the n-type cladding layer satisfies a relationship of xn<xp.